Tag Archives: gas giants

I recently came across an article with the title of combustible ice, also called “fire ice.” I realize that anything can be made to combust. I never thought of ice doing that. My next thought was that ice might mean something else than frozen water. Diamonds are referred to as ice because of their ability to transfer heat. The United States Immigration and Customs Enforcement (ICE) can be a hot topic in some quarters. Snow Crash has linked frozen water with the mes collected by Enlil. As you can see I was very curious about the article.

It turns out the ice referenced in this particular article is hydrocarbons frozen in ice crystals. It is natural gas (mostly methane) that has been trapped in the crystalline structure of water as it froze. We can easily imagine the liquid methane atmosphere of Neptune or methane sheets and snow on Makemake (a newly discovered plutoid in the Kuiper Belt) and Eris, but it is not something we think about on earth.

Methane needs to be below -297 degrees Fahrenheit for it to become solid, but because it is inside the ice, the “fire ice” can remain stable at much higher temperatures (around 29 degrees Fahrenheit). Methane is in the atmosphere of all the gas giants in our solar system and might be found in ice form on the dwarf planets like Pluto (we’ll have to wait for the New Horizon’s fly by in 2015. None of the extrasolar planets seem to have methane in their atmosphere (although HD 209458 b might have water vapor in its atmosphere).

The “fire ice” on Earth usually forms deep under the surface of the oceans – down hundreds of meters into the dark depth. That’s not the only place on the Earth where it is. China has reported that it has found this “fire ice” in Qinghai Province.

Methane hydrates have been known since the 1960’s, but they have not been in the news much. You might ask why. The early known methane hydrates despots were deep on the ocean floor and mining for them was too expensive for what they could sell the methane for. With the current rise in the cost of fuel, the “fire ice” is looking more attractive. Japan plans to have a full scale mining operation up and running by 2016 and China is putting aside nearly a billion dollars on research of mining and using “fire ice”.

So how does “fire ice” differ from the run of the mill natural gas? Well, they don’t. Hydrates are routinely formed during the refining process. The hydrates can cause damage to the pipelines by blocking the flow. Ethylene glycol (antifreeze) can be used to stop the hydrates from forming.

Since methane hydrates are a form of natural gas, why are methane hydrates important? One reason is that it is natural gas. Natural gas is used primary in electrical generation in the US. Natural gas burns cleaner then coal or petroleum. Another reason the “fire ice” can be important is in transportation. To ship natural gas around the world, it is common to change it from a gaseous form to a liquid. This, very logically, is called liquefied natural gas. It takes a lot of energy to cool the gas down to – 256 degrees Fahrenheit. The “fire ice” remains stable at much higher temperatures, -4 degrees Fahrenheit.

“Fire ice” might be cheaper to transport, but it will not be a “silver bullet” that solves all our energy needs. For that there is no single, easy answer.

Today’s guest blogger is Dr. Fritz Benedict. He is a Senior Research Scientist at UT-Austin’s McDonald Observatory in west Texas. He is also involved with NASA and is currently attending the Towards Other Earths conference in Portugal. Dr. Benedict will be giving a lecture at HMNS on October 27, at 6:30 p.m.

Hello from Portugal, where it is raining; water from the sky, and exciting new developments in the area of extrasolar planets.

All of this week, experts from around the world are sharing ideas and results about planets tens to hundreds of light years away. It is absolutely amazing how much we can determine about exoplanets: mass, size, composition of atmosphere, temperature and density.

But, these are all gas giants (like Jupiter) or even larger. The holy grail in this game is a planet that is very similar to Earth; a place with a surface gravity, atmosphere, and oceans like we enjoy. So, this conference (called Towards Other Earths) has a reason to exist. What will be required to find and characterize an earth-like planet tens of light years distant?

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.